Seriously Kay dont try it!
This came up for discussion in my anti natal classes, while they are well up for your trying all sorts of methods to bring on the baby, castor oil is not one of them!
Its something that was used years ago, but medical advances now show it not to be a good idea.
For starters it will make a mess, my midwife has had girls rush in before after there mothers got them to take it, and its not a pretty sight! were talking nasty explosive diareah that may not go away for some time. It also has a major dehydrating effect that makes labour a lot harder on you physically as everything just goes right through you and the dehydration makes you to weak and tired to push!
Please dont resort to caster oil!